Magnesium is a very light, yet strong metal and is finding increasing acceptance for metal die castings, particularly where weight savings are desired. In addition, its property of shielding electromagnetic radiation is causing it to be of interest as a replacement for plastics in applications such as computers and mobile telephones. However, it is a reactive metal and corrosion, whether general or by galvanic effects, is a major problem.
A number of methods for applying a protective anodic oxide film on magnesium material have been available. These have sought to imitate the well established processes available for coating aluminium and its alloys, however achieving the same result on magnesium articles has been extremely difficult.
The anodisation of aluminium and its alloys is often conducted in sulphuric acid in which the oxide layer formed is slightly soluble. As the film builds outwards from the metal substrate, its rate of build decreases, so ultimately there is an equilibrium point at which the rate of dissolution is equal to that of further film growth. The dissolution of the film causes the formation of pores through which the ionic migration necessary to the electrochemical oxidation of the metal takes place. Without these pores only very thin films would be possible. After the electrochemical oxidation process is complete, the pores are scaled. Sealing of anodised aluminium can be achieved with hot water or simple inorganic chemical solutions.
Clearly an analogous process involving magnesium would attempt to simulate these features. However, because of the tendency of the forming film to crack and break due to the imposed tensile stresses, there are complications. Also, the use of an acidic solution to anodise magnesium is fraught with serious difficulties as magnesium is rapidly attacked by most common acids. Therefore, anodisation of magnesium should preferably take place in alkaline solutions.
One method of anodising magnesium relies on this property to create a rough, very porous layer which may form a base for paint or other surface coatings to be applied afterwards. Commonly, such an anodic film may be formed in an electrolyte of high pH, containing alkali hydroxides. The process proceeds by means of sparking, which sparking forms a sintered ceramic oxide film as the metal substrate is coated.
However, the forming of a sintered ceramic oxide film, through sparking, is not always desirable as the film is often brittle, uneven, and/or lacks uniformity.
A number of proprietary methods for anodisation of magnesium exist which seek to avoid this problem and hence create a stronger and/or more uniform film.
In PCT/NZ96/00016 (WO 96/28591) (Barton) there is disclosed a viable procedure for anodising magnesium or magnesium alloys. It involves anodising the material in an ammonia containing electrolyte solution. The presence of some phosphate compounds in the solution is also disclosed. Enhancements of such a Barton procedure are disclosed in PCT/NZ98/00040 (WO98/42892) (MacCulloch et al).
For environmental reasons arising from the emanation of ammonia and also taking into account potential problems associated with the disposal of ammonia-containing electrolytes and process washings, a process is desirable beyond those aforesaid where no ammonia or ammonium salts are present in the electrolite. However, the absence of ammonium compounds imposes difficulties in the functioning of the process in the areas of anodic polarisation, repeatability and film quality.
In PCT/NZ01/00215 (WO 02/28838 A2) there is disclosed another viable procedure for anodising magnesium or magnesium alloys which do away with ammonia-containing electrolytes. This method includes anodising the magnesium material while it is immersed in an aqueous electrolyte solution having a pH above 9, and in the presence of a phosphate (or phosphate ions). The solution also preferably includes a buffering agent such as a tetra-borate to maintain the pH of the solution above 9. There are also described pre-treatment steps prior to anodising.
Whilst the methods and apparatus described in PCT/NZ01/00215 result in a viable procedure for anodising magnesium, the solutions contain boron (or a borate) which is not always desirable as it can be environmentally harmful if not disposed of properly after use. Furthermore, some of the pre-treatment steps described are somewhat involved. It would therefore be desirable if there was a viable procedure for anodising magnesium or magnesium alloys which used an electrolyte that preferable did not contain ammonia and/or boron/borate and/or did not require the use of such involved pre-treatment step(s).
Furthermore, many procedures for anodising magnesium necessarily involve the use of a pulsed DC current, which requires the use of specialised and expensive rectifiers. It would also be desirable therefore if there was available a viable procedure which produced the required or desired results using straight or flat waveform DC (referred to herein as “straight DC”).
It is therefore an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description that is given by way of example only.